1. Field of the Invention
This invention relates to display technology, and more particularly to the display of text information on a directed beam cathode ray tube.
2. Description of the Prior Art
Refresh displays may be broadly classified as raster, in which the beam is systematically moved to all points of the image and the information manifest by switching the beam on to light appropriate segments, or classified as directed beam, in which the beam is driven only along selected paths, the paths themselves portraying the information with beam switching supplemental. Directed beam is often called "stroke", but sometimes called "calligraphic", both refer to the "directed line" of penmanship. Raster is best at portraying solid areas of tone, such as in commercial TV, and stroke is best at portraying lines, as in graphics. A text display may be constructed with either technology.
Most directed beam displays operate to receive a sequence of positional coordinates and display a straight line between the present position and the new position. Such displays are also called vector stroke because the image is composed of discrete strokes, each a straight line vector. U.S. Pat. No. 3,659,282, issued Apr. 25, 1972, moves the beam between two arbitrary points in a series of tiny steps generated by a digital counter from a clock pulse source. To prevent positional staircasing, which would manifest in the image as a dotted line, a high frequency low-pass filter operates to smooth the tiny steps. U.S. Pat. No. 3,364,479, issued Jan. 16, 1968, achieves essentially the same result of generating straight lines by cascading a series of analog delays and summing equally the outputs. This yields a "boxcar averager" by which an impulse input is filtered to produce an output resembling a "square box", when viewed on an oscilloscope, as each delay outputs the impulse sequentially until the maximum delay is reached. In the display, as a new position is entered into the filter, the old position is still being output by the series delays. As time progresses, the new position is output by progressively more delay stages causing the command position to move uniformly in time to the new position. The command position reaches the new position at the maximum delay time, at which time a new coordinate is entered. In theory the same staircasing of the previous patent should result. However, the delay elements chosen provide sufficient low-pass filtering to obviate the need for a separate filter. U.S. Pat. No. 3,333,147, issued July 25, 1967, also operates to display a straight line using a boxcar averager, however in this case the desired function is approximated by a two-pole resonant low-pass filter. Because the beam does not move at a uniform rate, such systems often require a beam intensity control to modulate brightness in proportion to beam velocity to obtain uniform brightness vectors. U.S. Pat. No. 3,786,482, issued Jan. 15, 1974, generates the desired ramp functions accurately through the use of integrators implemented as current sources and capacitors. Integrators are special low-pass filters exhibiting a 20 db/decade attenuation slope for all frequencies. Two additional low-pass filters are shown at the output of the integrator, presumably to remove switching impulses produced by imperfections in the integrators.
Thus far, all prior art discussed has used a single deflection system. Such a system in a CRT must be either electrostatic deflection, which suffers from lower resolution, or magnetic deflection, which due to yoke inductance and resonances is limited to lower speed systems. A high-priced alternative for higher performance is illustrated in U.S. Pat. No. 3,437,869, issued Apr. 18, 1969. In this application two deflection systems are used. One is a high inductance, low-speed magnetic system and the second is a high-speed deflector capable of only very limited range. This second system is a low-inductance yoke in the above patent, but electrostatic micro-deflectors have also been used. In such a system operating, for example, as a text display, the beam center is slowly moved anywhere on the screen to character centers while the micro-deflectors rapidly form the characters around the centers. This system requires two complete sets of drivers, and effectively doubles the complexity of the logic circuits to dispatch tasks properly between the two drivers.
All of the above prior art has acted to produce straight-line vector strokes. U.S. Pat. No. 3,540,032, issued Nov. 10, 1968, uses a low-pass filter in series with the positional control signal and with a time constant about 1/5 the stroke rate. When both X and Y positions are updated simultaneously, the beam moves linearly to the new point, and in conjunction with intensity control circuitry a uniform straight line is displayed. However, if one direction, say the X-direction is updated first, followed by the Y-direction, initially the beam will move in the X-direction alone, followed by a time when both act in concert for a diagonal motion, eventually reaching a time when the X motion has terminated but the Y is still active, stroking a controlled curve on the image. The information required by the delay specification is similar to the introduction of an intermediate stroke, with each stroke changing only the target position of one axis.
As may be seen from this discussion of the prior art, low-pass filters are commonly used in stroke displays, their properties and actions within the system being the subject of several patents. However none have claimed or taught the use of a filter with fast attenuation beyond one-half the stroke frequency. All of the prior art systems assume the deflectors can reproduce the control signals with fidelity, limiting the speed of high resolution magnetic systems to frequencies much lower than the maximum driveable frequencies. At maximum driveable frequencies, magnetic deflection yokes exhibit multiple resonances, affecting gain and producing severe phase errors that are unreasonable to compensate. Such errors cause distortions of the vectors, such as spiraling, which cannot be compensated by merely moving the vector end points. The prior art presented three alternatives: (1) limited speed with a limited number of strokes per image, or (2) use of an electrostatic system with limited resolution and brightness, or (3) a costly dual deflector system was necessary.